DNA encoding human CV/2n superoxide dismutase muteins with at least serine or alanine at either or both residues 6 and 111

ABSTRACT

A polypeptide represented by the following general formula (I): ##STR1## wherein X 1  means a hydrogen atom, acetyl group of amino acid residue, X 2  and X 3  are either the same or different and mean individually an amino acid residue, and when X 2  stands for Cys, X 3  denotes an amino acid residue other than Cys.

This is a division of application Ser. No. 06/893,619, filed Aug. 6,1986 now U.S. Pat. No. 4,818,698.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to polypeptides analogous to human superoxidedismutase, which have a sequence of at least 153 amino acids andrepresented by the following general formula (I): ##STR2## wherein X₁means a hydrogen atom, acetyl group or amino acid residue, X₂ and X₃ areeither the same or different and mean individually an amino acidresidue, and when X₂ stands for Cys, X₃ denotes an amino acid residueother than Cys, and their preparation process, as well as their copper-and/or zinc-coordinated dimers represented by the following formula(II):

    2 [polypeptide (I).].Y.sub.1 Cu.sup.2+.Y.sub.2 Zn.sup.2+   (II)

wherein Y₁ and Y₂ stand individually for an integer of 0-4 and Y₁ +Y₂ is2 or 4.

2. Description of the Prior Art

Superoxide dismutase (hereinafter abbreviated as "SOD") is a compoundwhich was found for the first time by Fridovich, McCord, et al. J. Biol.Chem. 244, No. 22, 6049-6063 (1969) as an enzyme capable of convertingsuperoxides, intermediates in oxidations by xanthine oxidase, in thecourse of their investigation on xanthine oxidase. It can be purified bya variety of methods, for example, heat treatment [Japanese PatentPublication Nos. 39832/1970 and 48721/1974; Sugiura, et al., J. Pharm.Dyn. 4, 235-244 (1981); etc.], salting-out with ammonium sulfate andprecipitation in an organic solvent [Japanese Patent Laid-Open No.155991/1982; Stephen, A. G, et al., Biochimica et Biophysica Acta, 289,276-283 (1972)], gel filtration chromatography (Japanese PatentLaid-Open Nos. 102787/1981 and 10382/1982), affinity chromatography(Japanese Patent Laid-Open No. 121791/1983), etc. This SOD has drawnattention from the viewpoint of the oxygen toxicity protective mechanismin living bodies. It is now used as an anti-inflammatory for thetreatment of chronic arthrorheumatic osteoarthritis, radiation-inducedside effects, certain urosis and the like. Especially, bovine liver SODis used clinically.

In the meantime, the sequence of amino acids in human erythrocyteCu--Zn--SOD has been reported recently [Jabusch, et al., Biochemistry,19, 2310-2316 (1980); and Barra, et al., FEBS Letters, 120, 53-55(1980)]. There is also a report on the sequence of bases in a gene ofhuman-origin SOD (hereinafter called "h-SOD") [Sherman, et al., Proc.Natl. Acad. Sci. U.S.A., 80, 5465-5469 (1983)]. Production of h-SOD inEscherichia coli (hereinafter called "E. coli) and yeast by a geneticoperation has also been reported (Japanese Patent Laid-Open No.137286/1985).

In order to use SOD clinically especially as an anti-inflammatory or forother therapeutic purposes, it is essential thatphysiologically-acceptable SOD is supplied stably. To permit in vivoapplication of SOD in human bodies, SOD is required, in view ofpredictable immunological problems, to be h-SOD or at least an h-SODanalogous polypeptide in an immunologically acceptable class and also tobe a homogeneous enzyme. However, h-SOD has had a problem in its stablesupply.

SUMMARY OF THE INVENTION

With a foregoing in view, the present invention has as its object theprovision of novel h-SOD analogous polypeptides useful as medicines. Asa result, it has been found that recombinant h-SOD can be advantageouslyobtained when a genetic operation is used.

The recombinant h-SOD (I) has activities equal to or higher than nativeh-SOD. Unlike native h-SOD, charged isomers are not observed therein. Itis therefore a high-purity product having extremely high stability.Compared with native h-SOD, it is also extremely stable along thepassage of time in aqueous solutions or water-organic solvent solutions.In addition, it is free from side effects such as antigenicity. Therecombinant h-SOD (I) of this invention can therefore be usedeffectively, e.g., as anti-inflammatory and medicines for othertherapeutic purposes, as raw materials for clinical diagnoses, etc.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description and theappended claims, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the base sequence of the SOD polypeptide of a plasmidpSOD14 constructed in Example 1;

FIG. 2 shows the restriction endonuclease map of a DNA fragmentcontaining the h-SOD gene;

FIG. 3 depicts the h-SOD gene and its corresponding polypeptide composedof 153 amino acids;

FIG. 4 is a simplified flow chart of recombination for the constructionof a plasmid pSOD6;

FIG. 5 is a simplified flow chart of recombination for the constructionof the plasmid pSOD14;

FIG. 6 illustrates an electropherogram of an h-SOD analogous recombinantpolypeptide obtained in Example 1;

FIG. 7 is a simplified flow chart of recombination for the constructionof a plasmid pTJ102-SOD14; and

FIG. 8 shows an electropherogram of an h-SOD analogous recombinantpolypeptide obtained in Example 5 on the left side and that of thenative h-SOD on the right side.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

With a final target on h-SOD, the present inventors first of allcollected RNA from the tissue of a normal human liver which had beenenucleated. After obtaining poly(A)⁺ RNA, c-DNA was synthesized inaccordance with the routine procedure in genetic engineering. Subsequentto its recombination into a vector, E. coli was then transformed byusing the vector The E. coli was thereafter cultured, followed byscreening of a clone having the h-SOD gene from the resultant colony.From the above clone, DNA having the h-SOD gene was obtained by using asa probe a synthetic nucleotide corresponding to the 5-amino acid moiety,-5'ATGGCGACGAAGGCC3'-- of the N-terminal of h-SOD. By restrictionendonucleases Pst I and Pvu II, the gene was obtained as a 700 bp DNAfragment containing the h-SOD gene. Furthermore, a restriction enzymemap illustrated in FIG. 2 was obtained from the above DNA fragment byusing restriction endonucleases TthHB8I, Stu I, Hinf I, Rsa I, Fok I,Fnu 4HI and Sau 3AI. Based on the restriction enzyme map, the basesequence in the code region of the h-SOD polypeptide was provided by theMaxam-Gilbert method [Method Enzymol, 65, 449 (1979)], therebydetermining the polypeptide sequence of h-SOD and the base sequencecoding the polypeptide sequence as illustrated in FIG. 3. The presentinventors proceeded with a further research on the base sequence codingthe sequence of the amino acids in the h-SOD polypeptide with a viewtoward converting the Cys at the 111-site and/or 6-site (as counted fromthe Ala of the polypeptide) into other amino acid residues in accordancewith a site-specific mutagenesis method. As a result, a base sequencecapable of achieving this object was obtained It has then been foundthat recombinant polypeptides represented by the above formula (I) andhaving h-SOD activity (hereinafter called "recombinant h-SODs") andcopper- and/or zinc-coordinated recombinant h-SOD dimers represented bythe above formula (II) can be obtained by using the base sequence and E.coli in accordance with the routine procedure in genetic engineering. Ithas also been uncovered that the dimers (II) of the recombinant h-SODs(I) are obtained as homogeneous enzymes and are stable h-SOD-likepolypeptides having activities equal to or higher than h-SOD.

The present invention has been completed on the basis of these findings.

Among the recombinant h-SODs (I) of this invention, may be mentioned aspreferred examples those represented by the general formula (I) in whichX₂ is a neutral amino acid residue, e.g., Cys, Ser, Ala or Thr and X₃ isa neutral amino acid residue, e.g., Ser, Ala or Thr. Of these,particularly preferred are polypeptides of the general formula (I) inwhich X₁ is a hydrogen atom or an acetyl group, X₂ means Cys and X₃denotes. Ser. Among the recombinant h-SOD dimers (II), may be mentionedas preferred examples those represented by the general formula (II) inwhich Y₁ and Y₂ are both 2 and Y₁ and Y₂ are 4 and 0 respectively.

Polymers formed of these dimers (II) as their constituent units andhaving SOD activities are also included in the present invention.

For the preparation of each of the h-SODs (I) and their dimers (II) ofthis invention, a recombinant DNA is constructed from apolydeoxyribonucleic acid having a base sequence coding the amino acidsequence of polypeptide of the formula (I) and a replicative vector. Abacterial host is transformed using the recombinant DNA. Thetransformant is then cultured in a culture medium to produce the desiredpolypeptide gene, followed by collection of the desired product from thecultured cell broth.

In order to construct the recombinant DNA of the polydeoxyribonucleicacid, which has the base sequence coding the recombinant h-SOD (I), andthe replicative vector, it is only necessary to insert thepolydeoxyribonucleric acid coding the recombinant h-SOD (I) into a knownreplicative vector from the known DNA libraries described in the knownliterature referred to supra [Proc. Natl. Acad Sci. U.S.A., 80,5465-5469 (1983); Japanese Patent Laid-Open No. 137286/1985] by atechnique known in genetic engineering.

As to the technique known in genetic engineering, the above insertionmay be practised in accordance with a number of literature on geneticengineering, for examples, manuals of experiments such as:

(1) TAKAGI, Yasuyuki, "Idenshi Sosa Manual (Manual of GeneticOperations)", Kodansha;

(2) TAKAGI, Yasuyuki, "Idenshi Sosa Jikkenho (Laboratory Manual ofGenetic Operations), Kodansha;

(3) MANIATIS, T., et al., "Molecular Cloning: "Laboratory Manual", ColdSpring Harber Laboratory, Cold. Sp. Harb., U.S.A.; and

(4) WU, Ray, et. al., "Method in Enzymology", 101, Academic Press,U.S.A.

For example, it is only necessary to obtain the liver cDNA library froma human liver by molecular cloning with respect to h-SOD cDNA, totransform a bacterial host by the liver cDNA library, to conduct clonescreening on the resultant transformant by the colony hybridizationmethod to obtain an h-SOD-coding plasmid, and then to integrate theh-SOD-coding region into a producing vector. The outline of thisprocedure is shown in FIG. 4.

By subjecting the thus-obtained recombinant DNA (pSOD6), which is usedfor the production of h-SOD, to a site-specific mutagenesis method, itis possible to construct a replicative recombinant DNA having a basesequence coding the sequence of the constituent amino acids of therecombinant h-SOD (I) one or two amino acids of which are different fromthe amino acid sequence of h-SOD.

As the above site specific mutagenesis method, the following method maybe mentioned. Namely, BamHI was caused to act on pSOD6 of the closedcircle structure (cc) in the presence of ethidium bromide (EtBr) underlight-shielded conditions, whereby pSOD6 was converted into the opencircle structure (oc). Exonuclease III was then caused to act, wherebythe DNA at the site where mutagenesis was desired was converted into asingle strand form. The resultant synthetic nucleotide having themutable site was annealed to the partly single-strand plasmid. DNApolymerase and T₄ DNA ligase were then caused to act in the presence ofdNTP so that the synthetic nucleotide was incorporated in the plasmid["Experimental Manipulation of Gene Expression", 291-303 (1983),Academic Press]. The outline of this recombination procedure is shown inFIG. 5.

In the manner described above, the plasmid pSOD14 DNA containing theh-SOD gene having for example a base code TCC in which the Cys (TGC) atthe 111th site counted from Ala has been varied to Ser is obtained.

As illustrative synthetic nucleotides useful in the above-describedvariation method, various synthetic nucleotides given in the followingTable 1 may be mentioned. By using these synthetic nucleotides, theircorresponding plasmids are obtained.

                                      TABLE 1                                     __________________________________________________________________________    Amino acid                                                                          Synthetic nucleotide (codon)*                                                                  Raw material plasmid                                                                     Constructed plasmid                         __________________________________________________________________________    Cys   GAGACCAT|TGC|ATCATTGG                                 Ser   AGACCAT|TCC|ATCATTG                                                          pSOD6      pSOD14                                      Tyr   AGACCAT|TAC|ATCATTG                                                            "        pSOD35                                      Ala   GAGACCAT|GCC|ATCATT                                                          pSOD14     pSOD36                                      Phe   AGACCAT|TTC|ATCATTG                                                          pSOD6      pSOD37                                      Trp   AGACCAT|TGC|ATCATTG                                                            "        pSOD38                                      Arg   GAGACCAT|CGC|ATCATT                                                            "        pSOD39                                      Gly   GAGACCAT|GGC|ATCATT                                                            "        pSOD40                                      Pro   GAGACCAT|CCC|ATCATT                                                          pSOD14     pSOD41                                      Thr   GAGACCAT|ACC|ATCATT                                                            "        pSOD42                                      His   GAGACCAT|CAC|ATCATT                                                          pSOD35     pSOD43                                      Asn   GAGACCAT|AAC|ATCATT                                                            "        pSOD44                                      Asp   GAGACCAT|GAC|ATCATT                                                            "        pSOD45                                      Leu   GACCAT|TTG|ATCATTGG                                                          pSOD37     pSOD46                                      Ile   GAGACCAT|ATC|ATCATT                                                            "        pSOD47                                      Val   GAGACCAT|GTC|ATCATT                                                            "        pSOD48                                      Gln   GACCAT|CAA|ATCATTGG                                                          pSOD43     pSOD49                                      Lys   GACCAT|AAA|ATCATTGG                                                          pSOD44     pSOD50                                      Glu   GACCAT|GAA|ATCATTGG                                                          pSOD45     pSOD51                                      Met   GAGACCAT|ATG|ATCATT                                                          pSOD46     pSOD52                                      __________________________________________________________________________     *between the dashed lines.                                               

Other methods described in literature may also be applied besides theabove-mentioned site-specific mutagenesis method [Proc. Natl. Acad. Sci.U.S.A., 81, 4008 (1984); Nucl. Acid. Res., 10, 6487 (1982)].

In order to convert, for example, the Cys at the 6-site counting fromthe Ala into another amino acid, it is only necessary to isolate afragment of an h-SOD analogous gene, for example, from pSOD14, to splitat an appropriate site downstream the TGC coding the 6-site Cys with arestriction endonuclease, and then to bind a synthetic nucleotide havinga codon coding an amino acid other than Cys (TGC), for example,5'-TGTCCGTGCTGAAGGG-3' having the codon TCC of Ser. Plasmid pSOD53prepared above by way of example has a code capable of producing arecombinant h-SOD in which the Cys at each of the 6-site and 111-site ofh-SOD has been replaced by Ser.

As host cells preferable for the cloning and/or replication of genescoding the recombinant h-SOD (I) of this invention, may be mentionedthose capable of being grown in cultures of fermentation out ofprocaryotes such as bacteria and eucaryotes such as Yeast and monkeykidney cells. Particularly preferred are E. coli, Bacillus subtilis,Saccharomyces cerevisiae, Streptomyces, and COS cells originated frommonkey kidney cells.

As the above vector, vectors of various kinds usable in theabove-described cells may be used. They can be derived from plasmids andvirus as needed. Vectors function for cloning and/or replication. Thereare a number of literature on vectors and numerous vectors arecommercially available. These vectors generally contain markers by whichtheir screening is feasible. There are cytotoxic resistance,auxotrophism and the like as these markers. A single vector may oftencontain many vectors which impart different characteristics. In the caseof a cloning vector, there are both control signals of transcriptionstart and transcription stop.

As such advantageously usable vectors, may be mentioned, for example,vectors containing 1 pp promoters such as pIN-I, pIN-II and pIN-III(PKEN vectors), vectors containing pho5 promoters such as pAM82, SV40vectors such as plasmid pSV2, and so on.

When yeast is employed as the host organism for the expression, the DNAwhich is useful in the practice of this invention is not necessarilylimited to pTJ102-SOD14 employed in Examples, which will be describedsubsequently, but any yeast vectors may also be applicable provided thatthey contain a promoter functional in yeast and a DNA sequence capableof being transcribed by the above-mentioned functional promoter to anm-RNA from which h-SOD or an h-SOD derived polypeptide described in thisspecification can be translated. The yeast vector may be either any oneof the vectors in the three types [YI_(p), YE_(p) and YR_(p) types]classified by Struhl, et al. [Struhl, K., Stinchcomb, D. T., Sherer, S.,Davis, R. W.: Proc. Natl. Acad. Sic. U.S.A., 76,1035(1979)] or any oneof the YC_(p) type vectors classified by Clarke, et al. [Clarke, L.,Carbon, J.: "Nature", 287, 504(1980)]. As a further alternative, it maybe a vector which is a combination of more than one type of vectors likepAM82. As a still further alternative, the yeast vector may be a shuttlevector such as pAM82, which additionally carries a DNA sequence requiredfor the replication and maintenance of the plasmid in E. coli.

The promoter may be chosen not only from Pho5 promoter but also fromADH1 promoter [V. M. Williamson, J. Bennetzen, E. T. Young, K. Naysmith& B. D. Hall: Nature, 283, 214(1980)], Ga110 promoter [St. John, T. P.,Davis, R. W.: "Cell", 16, 443(1979)] and other known promoters.Promoter-screening methods have also been reported, for example, by AkioTohe in "Kagaku to Seibutsu (Chemistry & Living Organism)", 21, 183-193(1983). A DNA with a promoter sequence may hence be obtained, forexample, by following the screening method introduced by him.

In addition, it should also be noted that the yeast strain useful in thepractice is not necessarily limited to AH22 strain. Any strains whichbelong to Saccharomyces corevisiae may basically be used. If anone-YI_(p) type vector is used, it is however necessary to introduce itinto a host strain which has an auxotrophic mutation like leu²⁻ mutationin AH22 strain. Accordingly, the vector is supposed to have a DNAsequence capable of complementing the above mutation. This is one ofroutine techniques practiced for the stable maintenance of plasmids ontheir corresponding host cells. Yeasts other than S. cerevisiae may alsobe employed to achieve expression of h-SOD and the h-SOD derivativesdescribed in the present specification, provided that yeast vectorsappropriate to such other yeasts are used.

As a culture medium useful in obtaining the recombinant h-SOD (I) fromthe resultant transformant, known culture media employed for thispurpose may be mentioned. Preferred are culture media containingsuitable amounts of copper and/or zinc ions therein.

The recombinant h-SOD (I) of this invention can be obtained by culturingthe transformant at 30°-42° C., preferably around 37° C., for 3-48 hoursby a known method, for example, the aerated stirring culturing method,shaking culturing method, rotation culturing method, standstillculturing method or the like. Where the recombinant h-SOD (I) isreplicated within cells, the cells are separated by centrifugation whenthe cultured cell broth has reached a high concentration. The cells arelysed, followed by separation and purification of the recombinant h-SOD(I) by the various methods described in the above-described literature,for example, extraction, ion exchange chromatography, affinitychromatography, electrophoresis or dialysis or a combination thereof.Where the product is secreted, the culture medium is processed in asimilar manner.

According to the above-described process of this invention, therecombinant h-SOD (I) is obtain first of all as a copper- and/orzinc-coordinated dimer represented by the formula (II). The recombinanth-SOD (I) can however be obtained with ease provided that the dimer istreated in a manner known per se in the art.

Having generally described the invention, a more complete understandingcan be obtained by reference to certain specific examples, which areprovided herein for purpose of illustration only and are not intended tobe limiting unless otherwise specified.

EXAMPLES

The present invention will hereinafter be described by the followingReferential Examples and Examples. It should however be born in mindthat this invention is not necessarily limited to or by them.

REFERENTIAL EXAMPLE 1 Preparation of h-SOD poly(A)⁺ RNA

(i) After homogenizing 5 g of normal human liver tissue in 25 ml of 4Mguanidine isothiocyanate, the homogenate was maintained at 60° C. Phenolin the same volume was then added and mixed. The mixture was passedthrough an 18-gauge injection needle, 10 times, so that the DNA wassplit and the viscosity was reduced, followed by addition of 12.5 ml ofa liquid mixture consisting of 0.1M sodium acetate, 10 mM Tris-HCl (pH7.4) and 1 mM EDTA and 25 ml of chloroform-isoamyl alcohol (24:1). Theresultant mixture was stirred at 60° C. for 10 minutes, cooled in iceand then centrifuged at 10,000 rpm for 10 minutes, thereby obtaining awater layer. The same volume of chloroform-phenol was added to the waterlayer. The resultant mixture was stirred at 60° C. for 10 minutes andthen cooled in ice. By centrifugation at 10,000 rpm for 10 minutes, awater layer was obtained. After extraction for removal of the phenoltwice with the same volume of chloroform-isoamyl alcohol, 2 volumes ofethanol were added and the resultant mixture was allowed to stand at-20° C. for 2 hours. The precipitate was collected by centrifugation at10,000 rpm for 30 minutes, dissolved with 25 ml of a solution containing0.1M Tris-HCl (pH 7.4), 50 mM NaCl, 10 mM EDTA, 0.2% SDS (sodiumdodecylsulfate) and 5 mg "Proteinase K" (protease; product of Merck &Co., Inc.), incubated at 37° C. for 1.5 hours. The temperature wasraised to 60° C., followed by addition of 12.5 ml of phenol and 12.5 mlof chloroformisoamyl alcohol. After stirring the resultant mixture at60° C. for 10 minutes, it was cooled in ice and centrifuged at 10,000rpm for 10 minutes to obtain a water layer. The water layer was treatedagain with phenol, followed by extraction of the chloroform twice. Afteran addition of 2 volumes of ethanol, the resultant mixture wascentrifuged and the precipitate was collected and was then dried underreduced pressure. In the above manner, 4.04 mg of crude RNA wasobtained.

(ii) After dissolving the above-obtained precipitate in 5 ml ofsterilized water, the solution was incubated at 65° C. for 5 minutes andthen chilled quickly, followed by addition of 5 ml of a solutioncontaining 40 mM Tris-HCl (pH 7.6), 1.0M NaCl, 2 mM EDTA and 0.2% SDS.The entire volume of the resultant mixture was caused to pass foradsorption through a column packed with 0.2 g oligo(dT)-cellulose(product of Colaborative Company) which had been equilibrated by amixture of 20 mM Tris-HCl (pH 7.6), 0.5M NaCl, 1 mM EDTA and 0.2% SDS.After washing the column with 10 ml of the same solution, RNAs otherthan poly(A)⁺ RNA were eluted first of all with an eluant consisting of5 ml of 20 mM Tris-HCl (pH 7.6), 0.1M NaCl, 1 mM EDTA and 0.1% SDS.Then, poly(A)⁺ RNA was eluted with an eluant consisting of 10 mMTris-HCl (pH 7.5), 1 mM EDTA and 0.05% SDS. The eluate was fractionated1 ml by 1 ml. By measuring the OD₂₆₀ value, Fraction No. 2 - FractionNo. 8 were collected as poly(A)⁺ RNA fractions. To those fractions, 1/10volume of 2.5M sodium acetate and 2 volumes of ethanol were added. Afterallowing the resultant mixture to stand overnight at -20° C., it wascentrifuged at 25,000 rpm for 20 minutes to obtain a precipitate and theprecipitate was then dried under reduced pressure. In this manner, 14.4μg of poly(A)⁺ RNA was obtained.

REFERENTIAL EXAMPLE 2 Synthesis and cloning of cDNA

(i) The synthesis and cloning of cDNA were conducted in accordance withthe Okayama-Berg method [Mol. Cell. Biol. 2, 161 (1982)]. The poly(A)⁺RNA (114.4 μg) obtained in Referential Example 1 was dissolved in 100 μlwater. A 4 μl aliquot of the resultant solution was transferred in amicrotube and then dried under reduced pressure. It was then dissolvedwith 10 μl of 5 mM Tris-HCl (pH 8.3) and heated at 65° C. for 5 minutes.After lowering its temperature to 37° C., 20 μl of a solution containing50 mM Tris-HCl (pH 8.3), 8 mM MgCl₂, 30 mM KCl, 0.3 mM dithiothreitoland 2 mM dNTP (a mixture of dATP, dGTP, dCTP and dTTP in the sameamounts), 10 μCi 1.4 μg vector primer DNA (product of PL-BiochemicalCompany), and 5 units reverse transcriptase (product of Life ScienceCompany) were added and then reacted at 37° C. for 20 minutes. Thereaction mixture was thereafter added with 2 μl of 0.25M EDTA and 1 μlof 10% SDS to terminate the reaction, followed by a treatment with 20 μlphenol-chloroform. To a water layer which had been obtained bycentrifugation, the same volume of 4M ammonium acetate and 80 μl ofethanol were added and the resultant mixture was left over at -70° C.for 15 minutes. A precipitate obtained by centrifugation at 15,000 rpmfor 10 minutes was dissolved in 10 μl of a mixed solution (mayhereinafter be abbreviated as "TE") of 10 mM Tris-HCl (pH 8.0) and 1 mMEDTA. The ethanol precipitation was repeated again, and after washingonce with 50 μl of 75% ethanol, the precipitate was dried under reducedpressure. It was then dissolved with 15 μl of a solution containing 140mM sodium cacodylate, 30 mM Tris-HCl (pH 6.8), 1 mM CoCl₂, 0.1 mMdithiothreitol, 0.2 μg of "Poly A" (product of Sigma Company) and 10μCi[α-³² P]dCTP. While incubating the resultant solution at 37° C., 18units of a terminal transferase (product of PL-Biochemical Company) wereadded. They were reacted at 37° C. for 20 minutes, phenol-chloroformtreated, ethanol precipitated, ethanol washed, dried and then driedunder reduced pressure. The thus-obtained product was dissolved with 10μl of a solution containing 50 mM NaCl, 10 mM Tris-HCl (pH 7.4), 10 mMMgCl₂ and 1 mM dithiothreitol, added with 2.5 units of "Hind III"(product of Takara Shuzo Co., Ltd.), and incubated at 37° C. for 1 hour.After chloroform-phenol treatment, ethanol precipitation, ethanolwashing and drying, the resultant precipitate was redissolved in 10 μlof TE, added with 3 μl of ethanol and then stored at 20° C. A solutionwhich had been prepared by dissolving 7 ng of an oligo(dG)integratedlinker DNA (product of PL-Biochemical Company) with 10 μl TE (pH 7.5)containing 0.1M NaCl was added to a 1 μl aliquot of the above-obtainedsolution. The resultant solution was incubated at 65° C. for 5 minutesand at 42° C. for further 30 minutes and was then chilled to 0° C.Thereafter, the solution was brought to contain 20 mM Tris-HCl (pH 7.5),4 mM MgCl₂, 10 mM (NH₄)₂ SO₄, 0.1M KCl, 0.1 mM β-AND, 50 μg/ml BSA, and0.6 μg E. coli DNA ligase (product of PL-Biochemical Company) and waterwas added to bring the total volume to 100 μl. The resultant mixture wasthen left over overnight at 12° C. After adding 1 μl of 4 mM dNTP, 1 μlof 15 mM β-AND, 1 μl of E. coli DNA ligase (0.4 μg; product ofPL-Biochemical Company), 1 μl of DNA polymerase I (0.3 μg; product ofPL-Biochemical Company), 1 μl of E. coli RNaseH (1 unit; product ofPL-Biochemical Company), the resultant mixture was incubated at 12° C.for 1 hour and at 25° C. for further 1 hour.

(ii) E. coli DH1 [J. Mol. Biol., 166, 557(1983); an E. coli strainsupplied courteously from the Genetic Stock Center, School of Medicine,Yale University (Stock No. CGSC Strain 6040)], which had been culturedin 100 ml BHI culture medium (product of DIFCO Company; a bovinebrain-heart extract medium) and was in the logarithmic phase of growth,was harvested and then suspended in 40 ml of an ice-chilled solution (pH5.8) of 30 mM potassium acetate, 100 mM RbCl, 10 mM CaCl₂, 50 mM MnCl₂and 15% glycerin. After allowing the suspension to stand at 0° C. for 5minutes, the cells were collected by centrifugation, followed by theirsuspension in a solution (pH 6.5) of 4 ml of 10 mM MOPS buffer (productof Dotai Company), 75 mM CaCl₂, 10 mM RbCl and 15% glycerin. Thesuspension was left over at 0° C. for 15 minutes to obtain competentcells.

(iii) A 20 μl aliquot of the DNA solution prepared in (i) was added to200 μl of the E. coli suspension and the resultant mixture was allowedto stand at 0° C. for 30 minutes. The mixture was heat-treated at 42° C.for 90 seconds, to which 800 μl of the LB (Luria-Bertani) culture medium(10 g bactotrypton, 5 g bactoyeast extract, 10 g NaCl, 1 l water; pH7.5), followed by incubation at 37° C. for 90 minutes. Its 100 μlaliquot was spread on an LB agar plate containing 50 μg/ml ampicillinand was cultured overnight to obtain a transformant.

REFERENTIAL EXAMPLE 3 Screening of h-SOD clone

The screening of the h-SOD gene clone was conducted on the transformant,which had been obtained after the overnight culturing, by using thecolony hybridization method. After transferring onto a nitrocellulosemembranes the colonies grown on the LB agar, the membranes were placedon LB agar media containing chloramphenicol (100 μg/ml) with thecolony-bearing side up. The membranes were incubated overnight at 37° C.to amplify the plasmids DNA in the cells. After treatment of themembranes for 5 minutes in 0.5N NaOH, for 3 minutes in 1M Tris-HCl (pH7.4) and for further 5 minutes in 0.5M Tris-HCl (pH 7.5)-1.5M NaCl, themembranes were dried at 80° C. for 3 hours. Then, the membranes wereenclosed in a polyvinyl chloride bag and was subjected to a washingtreatment at 60° C. for 15 minutes by using 30 ml of threefold SSC(26.28 g NaCl, 13.23 g sodium citrate, 1 l water) and 0.1% SDS. Thistreatment was repeated further twice. After the washing treatment, thefilm was dipped in threefold SSC containing 60 μg/ml salmon DNA, tenfoldDenhalt solution (50 g ficoll, 50 g polyvinylpyrrolidone, 50 g bovineserum albumin, 500 ml water) and 0.1% sodium pyrophosphate and allowedto stand overnight at 60° C. After washing the membranes twice with 30ml of a solution consisting of fourfold SSC (35.04 g NaCl, 17.64 gsodium citrate and 1 l water), 10 times of the Denhalt solution and 0.1%sodium pyrophosphate, it was hybridized overnight at 43° C. by using asynthetic nucleotide, 5'ATGGCGACGAAGGCC3' as a probe, labeled at the 5'end with ³² P (10⁷ cpm/μg). The synthetic nucleotide had the basesequence coding the N-terminus 5-amino acid moiety (Met, Ala, Thr, Lys,Ala) of h-SOD. After washing it twice, each for 15 minutes, at roomtemperature with fourfold SSC, tenfold of the Denhalt solution and 0.1%SDS, the membranes were dried and then subjected to autoradiography. Asa result of screening of the transformants in the above-describedmanner, a clone having the h-SOD gene was obtained. It was named"pSOD1".

REFERENTIAL EXAMPLE 4 Determination of the base sequence of the h-SODgene

After preparing pSOD1 DNA in a large amount by the lysozyme-SDS methodand cesium chloride-ethidium bromide method [Maniatis, et al.,"Molecular Cloning", 86-94, Cold Spring Harber (1982)], DNA fragments ofabout 700 bp containing the h-SOD gene was obtained by using restrictionendonucleases "Pst I" and "Pvu II" (product of Takara Shuzo Co., Ltd.).Using restriction endonucleases "TthHB8I", "StuI", "HinfI", "RsaI","FokI", "Sau3AI" (which are all products of Takara Shuzo Co., Ltd.) and"Fnu4HI" (NEB), a restriction map of the above fragment was prepared.The restriction map is illustrated in FIG. 2. Based on this restrictionendonuclease map, the base sequence of the code region of the h-SODpolypeptide was determined by the Maxam-Gilbert method [Method Enzymol,65, 449 (1979)]. Results are shown in FIG. 3. From the base sequence,the h-SOD gene was found to code 154 amino acids.

REFERENTIAL EXAMPLE 5 Expression of h-SOD in E. coli

(i) Using the EcoRI and BamHI sites of pIN-I-A2 which is one ofexpression vectors for E. coli [the same vector as pKEN 039 described inJapanese Patent Laid-Open No. 140800/1982; EMBO, J., 6, 771-775 (1982);obtained from State University of New York, 4.9 Kbp Amp⁴), theexpression of h-SOD in E. coli was attempted based on the base sequenceof the h-SOD gene determined in Referential Example 4. To 2 μl (0.1 μg)of a solution containing a 700 bp DNA fragment obtained by digestion ofPstI and PvuII, 2 μl of 1M Tris-HCl (pH 8.0), 2.4 μl of 0.1M MgCl₂, 2 μlof 1M NaCl, 0.8 μl of 0.3M dithiothreitol, 2 μl of 5 mM dNTP, 3 units T4of DNA polymerase (product of Takara Shuzo Co., Ltd.) and water wereadded to bring the total volume to 50 μl. After incubation of themixture at 37° C. for 10 minutes, it was subjected to aphenol-chloroform treatment and ethanol precipitation, and theprecipitate was washed and then dried under reduced pressure. It wasdissolved with 70 μl of water, followed by addition of 6.5 μl of 1MTris-HCl (pH 7.6), 10 μl of 0.1M MgCl₂, 10 μl of 10 mM ATP, 1.5 μl of0.3M dithiothreitol, 1 μl of 5'-phosphated BamHI linker [1 μg CGGATCCG(product of Takara Shuzo Co., Ltd.) per μl] and 2 units of T4 DNAligase. The resultant mixture was allowed to stand overnight at 22° C.,followed by a phenol-chloroform treatment and ethanol precipitation. Theprecipitate was dissolved in 70 μl of an aqueous solution whichcontained 1 μg of a cloning vector, pACYC184 DNA [Journal ofBacteriology, 134, 1141 (1978); ATCC 37033] digested with "BamHI"(product of Takara Shuzo Co., Ltd.) and treated with 0.6 unit bacterialalkaline phosphatase (hereinafter abbreviated as "BAP"; product ofTakara Shuzo Co., Ltd.). To this solution 10 μl of tenfold ligationbuffer [0.5M Tris-HCl (pH 7.4), 0.1M MgCl₂, 0.1M dithiothreitol, 10 mMspermidine and 10 mM ATP] and 1 unit of T4 DNA ligase were added, waterwas added to bring the total volume to 100 μl, and the resultant mixturewas then allowed to stand overnight at 4° C. After phenol-chloroformtreatment, ethanol precipitation and washing and drying under reducedpressure, the resultant product was dissolved in 10 μl TE (pH 8.0).Using the thus-obtained DNA, the E. coli DH1 was transformed followingthe procedures (ii) and (iii) of Referential Example 2. A plasmid, fromwhich DNA fragments containing the SOD gene were able to be cut out bydigestion with BamHI, was obtained. The plasmid was named "pSOD5".

(ii) Water was added to 150 μl (100 μg) pSOD5 DNA, 18 μl tenfold BamH Idigestion buffer [100 mM Tris-HCl (pH 8.0), 70 mM MgCl₂, 1M NaCl, 10 mMdithiothreitol] and 180 units BamH1 to bring the total volume to 180 μland was then digested at 37° C. for 3 hours. A gel slice containingabout 700 bp DNA fragments were cut out by electrophoresis in a 0.7%agarose gel. After extracting the DNA from the gel, it was subjected toa phenol-chloroform treatment for its purification, ethanol precipitatedand washed, and then dried under reduced pressure. The DNA was dissolvedin 44 μl TE (pH 8.0), to which 10 μl tenfold TthHB8I digestion buffer[100 mM Tris-HCl (pH 7.5), 100 mM MgCl₂, 1M NaCl, 10 mM dithiothreitol]and 10 μl (80 units) TthHB8I (product of Takara Shuzo Co., Ltd.) wereadded. The DNA was hydrolyzed at 37° C. for 3 hours and about 600 bp DNAfragments were separated by electrophoresis in a 0.7% agarose gel. Afterpurifying the DNA fragments in the manner described above, they weredissolved in 100 μl water. The DNA fragments had cohesive ends, TthHB8Iand BamHI, at both termini thereof and contained, on the side ofTthHB8I, a base sequence coding the amino acids after the glutamic acidwhich is the 22th amino acid in the SOD polypeptide.

(iii) Besides, the following 12 oligo nucleotides were chemicallysynthesized for the expression of h-SOD. ##STR3##

To a 1 μl (1 μg) aliquot of each of these synthetic nucleotides, wereadded 5 μl of tenfold concentration linker kinase buffer [0.7M Tris-HCl(pH 7.6), 0.1M MgCl₂, 50 mM dithiothreitol], 1 μl of 50 mM ATP, 31 μl ofwater and 1 μl (10 units) of polynucleotide kinase (product of TakaraShuzo Co., Ltd.). The resultant mixture was incubated at 37° C. for 1hour and at 85° C. for further 10 minutes and the cooled gradually. To a25 μl aliquot of this reaction mixture, 10 μl of tenfold ligationbuffer, 64 μl of water and 1 μl (2.8 units) of T4 DNA ligase (product ofTakara Shuzo Co., Ltd.) were added. The resultant mixture was allowed tostand overnight at 4° C. Thereafter, the mixture was phenol-chloroformtreated and the water layer was collected.

(iv) Besides, after incubating at 37° C. for 2 hours a liquid mixture of3 μl (2 μg) of pIN-I-A2 vector, 3 μl of tenfold BamHI digestion buffer,1 μl (20 units) of EcoRI, 2 μl (16 units) of BamHI and 21 μl of water, 5μl of Tris-HCl (pH 8.0), 60 μl of water and 5 μl (0.6 unit) of BAP wereadded. The resultant mixture was incubated at 37° C. for further 1 hour.Thereafter, it was phenol-chloroform treated and the water layer wascollected.

(v) Ten microliter aliquots of the thus-prepared DNA fragment solutioncontaining the DNA coding the TthHB8I-BamHI h-SOD gene, solutions of thejoined synthetic nucleotides and pIN-I-A2 vector were mixed, followed byaddition of 3 μl of 3M sodium acetate and 66 μl of ethanol. Afterallowing the resultant mixture to stand at -70° C. for 15 minutes, aprecipitate was collected by centrifugation. After drying it underreduced pressure, it was dissolved with 89 μl of water and 10 μl oftenfold ligation buffer and after an addition of 1 μl (2.8 units) of T4DNA ligase (product of Takara Shuzo Co., Ltd.), the mixture allowed tostand overnight at 4° C. After phenol-chloroform treatment, ethanolprecipitation, washing and drying, the DNA was dissolved in 10 μl TE (pH8.0), and E. coli DH1 which had been rendered competent in theabove-described manner was transformed by it. The cells were spread onthe LB agar medium containing 50 μg/ml ampicillin and then culturedovernight at 37° C. The thus-obtained h-SOD expression clone was named"pSOD6". The above steps are depicted in FIG. 4.

As a result of an EIA making use of an antibody against h-SOD, the E.coli strain DH1 which carried pSOD6 was found to express 2 μg/ml(cultured cell broth) h-SOD when cultured overnight in the BHI culturemedium.

(vi) Purification of h-SOD

After suspending 200 g of E. coli with pSOD6 expressed thereon in 600 mlof a solution of Tris-HCl buffer (pH 7.6) which contained 1 mM CuSO₄, 1mM ZnSO₄ and 50 mM saccharose, the resulting suspension was subjected toa supersonic treatment for 30 minutes by "Cell Disruptor 90" (tradename) manufactured by Branson, Inc. to lyse the cells. The lysate wasadded with 600 ml of a 3:5 mixed solvent of chloroform and ethanol andstirred at 4° C. for 15 minutes, and the resultant precipitate wasremoved by centrifugation. K₂ HPO₄ (300 g) was dissolved in thesupernatant and the resulting ethanol layer (500 ml) was chilled at -20°C. for 30 minutes. The resulting precipitate was discarded bycentrifugation and the supernatant was condensed under reduced pressurein an evaporator. The thus-obtained condensate (300 ml) was thensubjected to gel filtration by using a column (4.5×60 cm) packed with"Sephadex G-25 gel" (trade name; product of Pharmacia AB) which had beenequilibrated with a solution containing 50 mM saccharose and 25 mMphosphate buffer (pH 7.8), whereby the condensate was substituted forthe solution. Ten grams of DE52 (trade name; product of Whatman Company)were added and the resultant mixture was agitated at 4° C. for 30minutes, so that impurities were adsorbed and then removed on a glassfilter. After dialyzing the filtrate against 2.5 mM phosphate buffer (pH6.5) containing 50 mM saccharose, it was passed through a column (1.6×20cm) packed with "DEAE-Sepharose CL-6B" (trade name; product of FarmaciaAB) which had been equilibrated in advance with the same buffer, so thatthe recombinant h-SOD was adsorbed. The column was then washed with thesame buffer and the concentration of the phosphate buffer was increasedlinearly from 2.5 mM to 50 mM, thereby eluting the recombinant h-SOD.Since SOD activities were observed in two separate peaks, the eluatefraction corresponding to the first peak was collected and lyophilized.The thus-lyophilized powder was dissolved in 10 ml of distilled waterand poured in a column (4.5×80 cm) packed with "Sephadex G-100" (tradename; product of Farmacia AB) which had been equilibrated with 10 mMTris-HCl buffer (pH 7.0) containing 50 mM saccharose, whereby thelyophilized powder was purified by gel filtration. The activities of theh-SOD obtained in the above manner were 3,000-3,500 units per mg of SOD.It showed a single band upon its electrophoresis.

EXAMPLE 1 Conversion of Cys (111) of h-SOD to Ser

Site-specific mutagenesis was effected in accordance with theVlasuk-Inouye method ["Experimental Manipulation of Gene Expression",291-303, Academic Press (1983)].

(i) 5 μg cc (closed circular) DNA of pSOD6 (5.6 kb, Ap^(r)) which is aplasmid containing the h-SOD gene was dissolved with 300 μl of asolution containing 50 mM Tris-HCl (pH 7.5), 10 mM MgCl₂, 10 mM NaCl and0.15 mg/ml EtBr. Under light-shielded conditions, 200 units BamHI(product of Takara Shuzo Co., Ltd.) were added, followed by incubationat 37° C. for 90 minutes. 10 μl of 0.5M EDTA (pH 8.0) and 300 μl ofphenolchloroform were added and mixed. A water layer was collected bycentrifugation, to which 30 μl of 3M sodium acetate and 660 μl ofethanol were added. The resultant mixture was allowed to stand at -70°C. for 10 minutes, and a precipitate was collected by centrifugation anddried to solid under reduced pressure. It was then dissolved in 40 μl ofa solution containing 66 mM Tris-HCl (pH 8.0), 0.66 mM MgCl₂ and 10 mMdithiothreitol, followed by an addition of 50 units of exonuclease III(product of PL-Biochemical Company). The resultant mixture was incubatedat 37° C. for 90 minutes. After an addition of 2 μl of 0.5M EDTA (pH8.0) and phenol-chloroform treatment, the DNA was collected by ethanolprecipitation and was then dried to solid under reduced pressure. It wasdissolved in 50 μl of a solution containing 200 mM NaCl, 13 mM Tris-HCl(pH 7.6), 9 mM MgCl₂ and 10 mM dithiothreitol to obtain Liquid (1).Besides, 1 μg synthetic nucleotide 5' AGACCATTCCATCATTG 3' (note: the9-site C is G in the original SOD gene) was dissolved in 10 μl of asolution containing 50 mM Tris-HCl (pH 7.6), 10 mM MgCl₂, 5 mMdithiothreitol, 0.1 mM spermidine, 0.1 mM EDTA and 5 mM ATP, to which3.2 units of T4 nucleotide kinase (product of Takara Shuzo Co., Ltd.)were added. They were reacted at 37° C. for 1 hour to obtain Liquid (2).A 50 μl aliquot of Liquid (1) and a 5 μl aliquot of Liquid (2) weremixed, heated at 100° C. for 3 minutes and chilled quickly. Theresultant mixture was then left over at 4° C. for 2 hours, followed byaddition of 7 μl of 5 mM dNTP, 2 μl of 50 mM ATP, 3 μl (6 units) of T4DNA polymerase (product of Takara Shuzo Co., Ltd.), 3 μl (4.2 units) ofT4 DNA ligase (product of Takara Shuzo Co., Ltd.) and 30 μl of water.After allowing the resultant mixture to stand overnight at 12.5° C., itwas treated by phenol and ethanol precipitate was collected. The DNA wasthereafter dried to solid under reduced pressure. After dissolving theDNA in 10 μl of TE (pH 8.0), the E. coli strain DH 1 was transformed toobtain ampicillin resistant transformant.

(ii) About these transformants, a screening of clones hybridized withthe synthetic nucleotide which was labeled with ³² P at the 5'-end (10⁷cpm/μg) was carried out using the colony hybridization method. The cloneobtained in the above manner was named "pSOD14". Similar to theabove-described procedures, the base sequence of pSOD14 DNA was thendetermined by the Maxam-Gilbert method. As a result, it was confirmedthat the Cys-coding TGC had been changed to the Ser-coding TCC. The basesequence of the plasmid pSOD14 DNA is shown in FIG. 1.

(iii) The E. coli strain DH1 containing pSOD14, which had been obtainedin the above-described manner, was cultured overnight at 37° C. by therotation culturing method on the BHI culture medium (pH 7.4±0.2).

After completion of the culture, the cultured broth was centrifuged at15,000 rpm for 30 seconds to harvest the cells.

(iv) The collection of the recombinant h-SOD in its purified form fromthe above-obtained cells was conducted in accordance with the methodproposed by I. Fridovich, et al. [J. Biol. Chem., 244(22), 6049-6055(1969)]. Namely, after suspending 200 g of the cells in 600 ml of asolution containing 50 mM Tris-HCl buffer (pH 7.6) which contained 1 mMCuSO₄, 1 mM ZnSO₄ and 50 mM succharose, a supersonic treatment wasapplied for 30 minutes by "Cell Disrupter 900" manufactured by BransonCompany so that the cells were lysed. The lysate was added with 0.75volume of a 3:5 mixture of chloroform-ethanol and the resultant mixturewas stirred at 4° C. for 15 minutes and then centrifuged to remove theprecipitate. In the supernatant, dipotassium hydrogenphosphate wasdissolved to a concentration of 300 g/l so that the ethanol layer (about500 ml) was salted out. The ethanol layer was collected bycentrifugation and chilled at -20° C. for 30 minutes. The crystallizedprecipitate was removed by centrifugation and the supernatant (about 300ml) was concentrated under reduced pressure in an evaporator. Thethus-concentrated solution was subjected to gel filtration on a column(4.5×60 cm) packed with "Sephadex G-25" gel (product of Farmacia AB)equilibrated with 25 mM phosphate buffer (pH 7.8), whereby the solutionwas substituted for the 25 mM phosphate buffer. To the concentratedsolution, 10 g of DE52 (product of Whatman Company) was added and theresultant mixture was stirred at 4° C. for 30 minutes. Impurities wereadsorbed and the solution was then filtered. After dialyzing thefiltrate against 2.5 mM phosphate buffer (pH 6.5) containing 50 mMsaccharose, it was poured in a column (1.6×20 cm) packed with"DEAE-Sepharose CL-6B" gel (product of Farmacia AB) and equilibratedwith the same buffer, so that the recombinant h-SOD was adsorbed. Thecolumn was then washed with the same buffer and the concentration of thephosphate buffer was increased linearly from 2.5 mM to 50 mM, therebyeluting the recombinant h-SOD. Since SOD activities were observed in twoseparate peaks, these active fractions were separately pooled andlyophilized. Lyophilized powder of one of the two active fractions,which was eluted first, was dissolved in 10 ml distilled water and waspoured in a column (4.5×80 cm) packed with "Sephadex G-100" (product ofFarmacia AB) equilibrated with 10 mM Tris-HCl buffer (pH 7.0) whichcontained 50 mM saccharose, whereby the lyophilized powder was purifiedby gel filtration. The activities of the recombinant h-SOD obtained bythe above procedure were determined by the above-described methodproposed by Fridovich, et al. Per mg of SOD, 3000-3600 units wereexhibited. The recombinant h-SOD was at least equal in specificactivities and physicochemical properties to h-SOD purified from humanred blood cells.

This recombinant h-SOD was confirmed to be represented by the generalformula (II) in which X₁, X₂ and X₃ mean a hydrogen atom, Cys and Serrespectively and Y₁ and Y₂ are 2 respectively (measured by EIA andatomic absorption spectrophotometry.

(v) From 1 ml of overnight culture of E. coli strain DH1 carrying pSOD14[Escherichia coli DHI pSOD14; FERM BP-1696] in the BHI culture medium,cells were collected by centrifugation. After removal of thesupernatant, the cells were added with 1 ml of 50 mM Tris-HCl buffer (pH7.6) which contained 1 mM CuSO₄, 1 mM ZnSO₄ and 50 mM saccharose. Underice cooling, the resultant mixture was subjected to a supersonictreatment for 5 minutes ("Handy Sonic UR-20P" manufactured by TomyCorporation was used) so that the cells were lysed. A 1 μl aliquot ofthe lysate was dropped in the sample slots of an agarose gel film0product of Corning Company; "Universal") which had in advanceequilibrated by 20 mM tris-glycine buffer (pH 8.45). After conductingelectrophoresis at 250 V for 20 minutes in a 20 mM tris-glycine buffer(pH 8.45), the film was immersed for 2 minutes in a 5 mg/ml nitrobluetetrazolium solution which contained 0.1 mM riboflavin. The solution wasallowed to drip off from the film. After dipping the film for 1 minutein a 1% tetramethyl-ethylene diamine solution, the solution was allowedto drip off from the film. The film was caused to produce a color inwhite light until a contrast appears with a white background in a gel.After production of the color, the unreacted reagent was washed awaywith distilled water and the film was then dried. Results are shown inFIG. 6, in which an area having h-SOD activities did not produce thecolor but the other area was turned to a purple color. As apparent fromthis drawing, the width of the active band was narrow in the case of therecombinant h-SOD of this invention (lefthand side; Ser at the 111thsite of the amino acid sequence) which was obtained from the culturedcell broth of the E. coli strain carrying the plasmid pSOD14. It istherefore appreciated that the recombinant h-SOD is hard to yieldisomers and is also stable On the other hand, h-SOD obtained from E.coli carrying the plasmid pSOD6 had a broad band and produced manyisomers.

(vi) Stability of h-SOD and recombinant h-SOD on standing:

One milligram aliquots of h-SOD obtained in the procedure (vi) ofReferential Example 5 and the recombinant h-SOD obtained in Example 1were separately dissolved in portions of a 0.1M phosphate buffer (pH7.0) containing 50 mM of saccharose and then left over at roomtemperature for 1 week. The resulting solutions were subjected toisoelectric focusing. Each of h-SOD and the recombinant h-SOD showed asingle peak at an isoelectric point (pI) of 5.0 at the beginning of theexperiment. This peak will hereinafter be designated commonly as "S₀ ".As a result of the experiment, four isomers having different isoelectricpoints (pI=5.1, 4,9, 4.85 and 4.8) were formed. The peaks correspondingto these isomers will hereinafter be designated as S₋₁, S₁, S₂ and S₃respectively. The proportion of the starting h-SOD (S₀) dropped to about70%. In the recombinant h-SOD, an isomer was formed at pI=4.9. The peakcorresponding to this isomer will hereinafter be designated as " S₁. Theproportion of this isomer was however as low as 7.9% and more than 90%of the recombinant h-SOD was hence allowed to remain. Although S₁ hadcomparable activities with S₀, the activities of S₂ were lower thanthose of S₀. The activities of S₃ were less than one-half of those of S₀and S₋₁ had substantially no activities. Accordingly, significantimprovements in stability in both structure and enzymatic activitieswere confirmed as a result of the conversion of h-SOD to the recombinanth-SOD.

    ______________________________________                                        Compositions of h-SOD and                                                     recombinant h-SOD after 1 Week                                                Isomer     S.sub.0 S.sub.1 S.sub.2                                                                              S.sub.3                                                                             S.sub.-1                              pI         5.0     4.9     4.85   4.8   5.1                                   ______________________________________                                        h-SOD  0 hour  100.0%  0%    0%     0%    0%                                         1 week  71.0%   20.4% 4.0%   1.0%  3.5%                                Recom. 0 hour  100.0%  0%    0%     0%    0%                                  h-SOD  1 week  92.1%   7.9%  0%     0%    0%                                  ______________________________________                                    

EXAMPLE 2

In the same manner as in Example 1, pSOD35 was obtained by using anucleotide 5' GAGACCATACCATCATT 3' synthesized on the side. In addition,pSOD36 was also obtained from an experiment in which the syntheticnucleotide 5' GAGACCATGCCATCATT 3' was used on pSOD14 obtained inExample 1. In the h-SODs coded respectively by pSOD35 and pSOD36, theCys at the 111 site had been changed to Thr and Ala respectively. Theh-SODs produced by E. coli DH1 containing pSOD35 and pSOD36 respectivelywere both recombinant h-SODs stabilized as proteins and capable ofshowing SOD activities similar to Example 1.

Incidentally, recombinant h-SODs corresponding respectively to thesynthetic nucleotides shown above in Table 1 can be obtained if theabove procedures are practised by using synthetic nucleotides in steadof the above-described synthetic nucleotide.

EXAMPLE 3

To 100 μg (100 μl) pSOD14 DNA obtained in Example 1, were added 18 μl oftenfold BamHI digestion buffer [100 mM Tris-HCl (pH 8.0), 70 mM MgCl₂,1M NaCl, 10 mM dithiothreitol], 120 units (10 μl) BamHI (product ofTakara Shuzo Co., Ltd.) and 52 μl water. After reacting them at 37° C.for 2 hours, about 700 bp DNA fragments were extracted, purified andconverted into a dry state in the same manner as that described above byelectrophoresis in a 0.7% agarose gel. The thus-obtained DNA wasdissolved in 44 μl of TE (pH 8.0), followed by addition of 6 μl oftenfold TthHb8I digestion buffer [100 mM Tris-HCl, 100 mM MgCl₂, 1 mMNaCl, 10 mM dithiothreitol) and 80 units (10 μl) TthHB8I (product ofTakara Shuzo Co., Ltd.) so as to hydrolyze the DNA at 37° C. for 3hours. About 600 bp DNA fragments were isolated and purified in the samemanner as described above, followed by their dissolution in 100 μl ofwater. Besides, pSOD53 was also obtained in exactly the same manner asdescribed in Referential Example 5(v) by using 5' TGTCCGTGCTGAAGGG 3'and 5' CACGGACACGGCCTT 3' in lieu of 5' TGTGCGTGCTGAAGGG 3' and 5'CACGCACACGGCCTT 3' among the 12 strands of the synthetic nucleotidesemployed in Referential Example 5(iii). As a result of determination ofthe base sequence of the DNA of pSOD53 in the aforementioned manner, itwas confirmed that TGC, which had coded Cys at the 6th and 111th sites,had been both changed to TCC.

In view of results of an electrophoretic analysis in agarose, therecombinant h-SOD which had been obtained from the E. coli strain DH1carrying pSOD53 in the same manner as in Example 1 was found to bestable like the recombinant h-SOD produced by the E. coli strain DH1carrying pSOD14.

EXAMPLE 4

(i) An h-SOD gene cloning plasmid was prepared by modifying a cloningplasmid pSV2-dhfr [product of BRL Company; Mol. & Cell. Biol. 1,854(1981)] which contained the SV40 promoter of the gene of the mousefolic acid reductase. pSOD14 was digested by the restrictionendonucleases XbaII and BamHI to isolate the h-SOD gene Fragments ofthis gene were then ligated by T4 ligase with a vector which had beenobtained by digestion of pSP64 plasmid [product of Amasham Company;Nucleic Acid Res. 12, 7035 (1984)] with the restriction endonucleasesXbal and BamHI, thereby obtaining pSP64-h-SOD plasmid. By digestion ofpSP64-h-SOD with the restriction endonucleases HindIII and BamHI, DNAfragments containing the h-SOD gene were isolated again. Finally, theabove-mentioned HindIII-BamHI fragments were ligated with T4 ligase onthe vector which had been obtained by digestion of pSV2-dhfr plasmidwith HindIII and BglII so that a plasmid pSV2-h-SOD for the expressionof the h-SOD gene in mammalian cells was obtained.

(ii) The introduction (transformation) of the thus-obtained plasmid DNAin cells was done by the DNA-calcium phosphate precipitation method[TANPAKUSHITSU. KAKUSAN.KOSO (Proteins, Nucleid Acids & Enzymes),Special Edition, 27, 340(1984)]. Sterile air was blown into 1 ml of asolution which was composed of 20 mM HEPES buffer (pH 7.10), 50 mM NaCl,0.7 mM sodium phosphate, 120 mM CaCl and pSV2-h-SOD plasmid (10 μg/ml),so that a cloudy DNA-calcium phosphate solution was formed. It was thenadded to 50 Petri dishes (diameter: 8 cm), in which COS cells [ColdSpring Harbar, Symp. Quant. Biol., 44, 293(1979)] had been multiplied inDMEM medium+10% neonatal calf serum. After culturing the COS cells at37° C. for 48 hours in an incubator which contained 5% CO₂, the cellswere harvested and then suspended in 10 mM phosphate buffer (pH 7.4).The cells were thereafter disrupted by "Polytron" and then centrifugedat 15,000 rpm for 10 minutes to obtain a supernatant. The supernatantwas isolated by electrophoresis in agarose and a band corresponding toh-SOD was activated and stained to investigate its mobility. Results aresummarized in the following table.

    ______________________________________                                                        Mobility (cm)                                                                 Anode side                                                                            Cathode side                                          ______________________________________                                        Supernatant of COS cells                                                                        2.2.sup.a)                                                                              0.9.sup.b)                                        transformed by pSV2-h-SOD                                                     SOD obtained from human                                                                         2.2.sup.a)                                                  red blood cells                                                               Supernatant of untreated    0.9.sup.b)                                        COS cells                                                                     ______________________________________                                         .sup.a) hSOD                                                                  .sup.b) Monkey SOC                                                       

EXAMPLE 5 Expression of h-SOD in yeast

(i) Preparation of cloning plasmid

(1) 1 μg plasmid pSOD14 of Example 1 and 1 μg pUC19 [purchased fromTakara Shuzo Co., Ltd; Yanisch-Perron, C., et al., Gene (1985) 33,103-119] were digested at 37° C. for 2 hours in separate containers byusing 50 μl XbaI buffer [6 mM MgCl₂, 6 mM Tris-HCl, 100 mM NaCl, pH 7.4]which contained the restriction endonucleases XbaI and BamHI (purchasedfrom Takara Shuzo Co., Ltd.; all restriction endonucleases and otherenzymes employed for DNA modification and ligation, which willhereinafter be referred to, were produced by the same company unlessotherwise specifically indicated), each, in an amount of about 10 units.The XbaI-BamHI digest of pUC19 were phenol treated, ethanolprecipitated, ethanol washed and then dried. The precipitate wasthereafter dissolved in 10 μl TE [10 mM Tris-HCl (pH 8.0)-1 mM EDTAsolution], to which were added 10 μl fivefold BAP buffer [250 mMTris-HCl, pH 8.3], 29 μl sterile distilled water and 1 μl (about 0.4unit) bacteria (E. coli) origin alkaline phosphatase [bacterial alkalinephosphatase] (hereinafter abbreviated as "BAP"). The resultant mixturewas incubated at 65° C. for 30 minutes (this is an enzymatic treatmentfor the removal of the phosphoric acid at the 5'-terminus and willhereinafter be called "BAP treatment"). The BAP-treated XbaI-BamHIdigest of pUC19 and XbaI-BamHI digest of pSOD14 were subjected toelectrophoresis in a 1% (w/v) agarose gel. The resultant gel was dippedin a solution which had been obtained by adding ethidium bromide at aconcentration of 0.5 μg/ml to an electrophoretic buffer [0.04MTris-acetate, 2 mM EDTA (pH 8.1)] (hereinafter called "EtBr stainingsolution"). The gel was left over for 20 minutes there. The gel was thenpulled out and placed on a table, where it was exposed tolong-wavelength ultraviolet rays (366 nml radiated from Model UVL-56manufactured by Ultraviolet Products Inc.) to visualize the DNAs.Judging from a DNA fragment size marker of the HindIII restrictionenzyme digest of λ phage DNA which was subjected concurrently toelectrophoresis, gel pieces which contained the XbaI-BamHI DNA of about2680 base pairs (hereinafter abbreviated as "bp") and XbaI-BamHI DNAfragments of about 700 bp respectively were sliced out by anatomicalscalpels from the electrophoresis lane of the pUC19 digest and pSOD14digest respectively. These gel pieces were placed in separate dialysistubes each of which contained about 0.4 ml of the above-describedelectrophoretic buffer. After closing both ends of each of the tubes,the tube was placed standstill in a horizontal electrophoretic bathfilled with the above-mentioned electrophoretic buffer. It was thensubjected to electric elution at 200 V for 30 minutes. One end of thedialysis tube was opened and the buffer containing the eluted DNA wastransferred from the tube into an Eppendorf tube.

The thus-obtained eluates, which contained these fragments, wererespectively phenol treated, ethanol precipitated, ethanol washed anddried. The resultant precipitates were separately dissolved in 10 μl TE.A 2 μl aliquot of the above-prepared solution, which contained about2680 bp fragments obtained from pUC19, was added to a 2 μl aliquot ofthe above-prepared solution of about 700 bp fragments obtained frompSOD14, 1 μl tenfold ligation buffer [described already in ReferentialExample 5] and 1 μl (about 350 units) T4 DNA ligase. The resultantmixture was allowed to stand overnight at 4° C. By using thethus-prepared ligation liquid, the E. coli strain DH1 was transformed inaccordance with the procedures (ii) and (iii) of Referential Example 2.

(2) Four transformants were chosen at random from the resultanttransformants. They were cultured overnight at 37° C. in a liquid BHIculture medium (ampicillin content: 50 μg/ml) and a plasmid was obtainedfrom the cultured cell broth in the following manner.

Namely, a 1.5 ml aliquot of the cultured cell broth which contained thecells cultured in the liquid medium was transferred in an Eppendorf tube(product of Eppendorf Company) and centrifuged at 15000 rpm and 4° C.for 30 seconds by a small centrifuge ("MR-15A", manufactured by TomySeiko K.K.), and the resultant supernatant was thrown away. To the cellscollected on the bottom of the tube, 100 μl lysozyme solution [20%glucose, 50 mM Tris-HCl (pH 8.0), 1 mM EDTA (pH 8.0); a solutionobtained by dissolving chicken egg white origin lysozyme (product ofSigma Company) in an amount of 2 mg per ml of the solution] was added tosuspend the cells. After holding the container standstill for 20 minutesin ice water, 0.2N NaOH and 200 μl 1% SDS (sodium dodecylsulfate) wereadded further. The resultant mixture was stirred thoroughly and thenleft over in ice for 5 minutes. Thereafter, 150 μl 3M sodium acetate (pH4.8) was added, and the resultant mixture was stirred thoroughly andleft over in ice for 3 hours. The mixture was then centrifuged at 15000rpm and 4° C. for 10 minutes by the above centrifuge and the resultantsupernatant was transferred in a separate Eppendorf tube. 1.2 ml ethanolwas added, mixed thoroughly, and then allowed to stand at -20° C. for 20minutes. The mixture was then centrifuged at 15000 rpm and 4° C. for 10minutes by the above centrifuge and the resultant supernatant was thrownaway. In order to wash the precipitate, 1 ml anhydrous ethanol was addedto the container. The resultant mixture was centrifuged at 15000 rpm and4° C. for 30 seconds by the above centrifuge and the supernatant wasagain thrown away. The liquid still remaining in the container andconsisting principally of ethanol was evaporated at room temperature for10 minutes under reduced pressure by a vacuum pump. A small amount ofsolid (which contained the plasmid DNA) occurred on the bottom of thecontainer was obtained, to which 50 μl TE (pH 8.0) was added to dissolvesame. A 2 μl aliquot of the TE solution was digested at 37° C. for 2hours in 20 μl XbaI buffer which contained 2.5 units XbaI, 2.5 unitsBamHI and 1 μg bovine spleen origin ribonuclease (product of FarmaciaAB; hereinafter abbreviated as "RNase"). The resultant digest wassubjected to electrophoresis in 0.7% agarose. As a result, theappearance of about 700 bp DNA fragment from each of the four plasmidswas confirmed. One of these four plasmids was named "pUC19 -SOD14".

The strain DH1 retaining this plasmid was named "DH1/pUC19-SOD14". Thisbacterium was cultured in a large quantity to obtain the DNA. Namely,DH1/pUC19-SOD14 was cultured overnight at 37° C. in 100 ml liquid BHIculture medium in which 50 μg/ml ampicillin was contained. The culturedcell broth was centrifuged at 5000 rpm and 4° C. for 10 minutes by using"No.17N Rotor" manufactured by Tomy Seiko K.K. and the resultingsupernatant was thrown away. Added to the centrifuge tube was 20 ml 20mM Tris-HCl (pH 7.5) to suspend the cells on the bottom. The suspensionwas transferred in a separate container, followed by its centrifugationat 5000 rpm and 4° C. for 10 minutes by the same "No.4 Rotor"manufactured by Tomy Seiko K.K. The supernatant was thrown away andcells were collected on the bottom. The cells were then suspended in 2ml lysozyme solution and the container was left over in ice for 30minutes. After adding 0.2N NaOH and 4 ml 1% SDS solution and thoroughlystirring the resultant mixture, the container was left over in ice for20 minutes. 3 ml 3M sodium acetate (pH 4.8) was then added and theresultant mixture was thoroughly stirred, and the container was leftover in ice for 2 hours. Using the above-mentioned "No.4 Rotor", theabove-prepared mixture was centrifuged at 14000 rpm and 4° C. for 10minutes and the resulting supernatant was transferred in a separatecontainer. Added to the supernatant were 2.5 volumes of anhydrousethanol. After cooling the resultant solution at -20° C. for 20 minutes,it was centrifuged at 14000 rpm and 4° C. for 10 minutes by the above"No.4 Rotor". The resulting supernatant was thrown away, and theprecipitate which occurred on the wall and bottom of the container wasdissolved in 4 ml TE, followed by an addition and dissolution of 4.2 gcesium chloride. 0.5 ml of 10 mg/ml ethidium bromide solution was addedfurther. The resultant mixture was transferred in a "Quick Seal"centrifuge tube manufactured by Beckman Company and its upper endopening was closed by a "Tube Sealer" manufactured by Beckman Company.It was then centrifuged at 50000 rpm and 15° C. for 16 hours by a "VTi65.2 Rotor" manufactured by the same company. The tube was gentlyremoved from the rotor and in a dark room, the second band from the topin the central part out of the bands observed in the orange color uponexposure to the long-wavelength ultraviolet rays was collectedseparately while the internal liquid was allowed to drip out through ahole formed by an injection needle through the bottom of the centrifugetube.

The liquid containing the above-collected band was added at roomtemperature with n-butanol in an amount of 1 volume to 3 volumes. Theresultant mixture was shaken and mixed to extract and remove ethidiumbromide intercalated in the DNA. After repeating this extraction andremoval operation 5 times, the lower layer, i.e., water phase wastransferred in a dialysis tube in order to get rid of cesium chloride.Using 500 ml TE as an outer solution, the water phase was dialyzed at 4°C. for 2-30 hours while replacing the outer solution 3 times in thecourse of the dialysis. The inner solution of the dialysis tube was thentransferred into an Eppendorf tube, to which the same amount ofTE-saturated phenol was added. The resultant mixture was stirredthoroughly and centrifuged at room temperature and 15000 rpm for 30seconds by a small centrifuge, and of the resultant two layers, theupper layer was collected separately. The same amount of diethyl etherwas added and the resultant mixture was stirred thoroughly. The upperether layer was then thrown away. This ether extraction (for the removalof phenol) was conducted three times in total. The remaining ether wasremoved by vacuum evaporation. Then, per 400 μl liquid, were added 10 μl1M MgCl₂ and 40 μl 5M potassium acetate, followed by a further additionof 1 ml anhydrous ethanol. The resultant solution was chilled andlyophilized at -110° C. for 10 minutes. It was then centrifuged at 4° C.and 15000 rpm for 10 minutes by a small centrifuge, the supernatant wasthrown away, 1 ml anhydrous ethanol was added and the resultant mixturewas centrifuged for 30 seconds under the same conditions. Theprecipitate was dried to solid under reduced pressure, thereby obtaininga solid matter. The solid matter was then dissolved in 400 μl TE. Inthis manner, pUC19-SOD14 plasmid was obtained in a large amount.

(3) Thereafter, 1 μg of the DNA of pAM82 (furnished courteously from Dr.Kenichi Matsubara, the Cytoengineering Center, Osaka University;described in Japanese Patent Laid-Open No. 36699/1984; FERM-P-8838) wasdigested by about 10 units restriction endonuclease PvuII at 37° C. for2 hours in 20 μl M buffer [10 ml Tris-HCl, 10 mM MgCl₂, 1 mM DTT and 50mM NaCl, pH 7.5). The resultant mixture was phenol treated, ethanolprecipitated, ethanol washed and then dried. The thus-obtainedprecipitate was digested again at 37° C. for 2 hours in 20 μl of Hbuffer (10 mM Tris-HCl, 10 mM MgCl₂, 1 mM DTT and 100 mM NaCl; pH 7.5)which contained about 10 units of the restriction endonuclease XhoI.After subjecting the digest to phenol treatment, ethanol precipitation,ethanol washing and drying, the resultant precipitate was dissolved in10 μl TE, to which 10 μl fivefold BAP buffer (described above), 29 μlsterile distilled water and 1 μl (about 0.4 unit) BAP were added. A BAPtreatment was then conducted at 65° C. for 30 minutes. The thus-treatedsolution will be called "pAM82 PvuII-XhoI/BAP solution". By about 10units of the restriction endonuclease SalI, a 1 μg aliquot ofpUC19-SOD14 plasmid DNA obtained before was digested at 37° C. for 2hours in 20 μl H buffer. After phenol treatment, ethanol precipitation,ethanol washing and drying, the digest was digested again by about 10units of the restriction endonuclease SmaI at 30° C. for 2 hours in 20μl SmaI buffer (10 mM Tris-HCl, 7 mM MgCl₂, 20 mM KCl, 7 mM2-mercaptoethanol, pH 8.0). The resultant digest and "pAM 82PvuII-XhoI/BAP solution" were respectively subjected to electrophoresisin a 1% agarose gel, whereby pUC19-SOD14 origin SalI-SmaI DNA fragmentsof about 700 bp and pAM82 origin PvuII-XhoI DNA fragments of about 10kilo bp were obtained by electric elution. After the eluates containingthese DNA fragments respectively were subjected to phenol treatment,ethanol precipitation, ethanol washing and drying, the resultingprecipitates were separately dissolved in 10 μl TE. 2 μl of each of thetwo types of DNA fragment solutions, 4 μl sterile distilled water, 1 μltenfold ligation buffer and 1 μl (about 350 units) T4 DNA ligase wereadded together and then allowed to stand overnight at 4° C. Using thisligation liquid, the E. coli strain DH 1 was transformed in accordancewith the procedures (ii) and (iii) of Referential Example 2. Two of thetransformants thus-obtained were cultured overnight at 37° C. in the BHIliquid culture medium which contained 50 μg/ml ampicillin. From thecultured cells, the solution containing the plasmid DNA was obtained inthe same manner as that described above. 2 μl aliquots of the solutionwere added separately to H buffer containing 2.5 units AatI and 1 μgRNase and to H buffer containing 2.5 units BamHI and 1 μg RNase. Each ofthe buffers was employed in such an amount that the final volume reached20 μl. The plasmid DNA was digested by the respective restrictionendonucleases at 37° C. for 2 hours. The resultant digests weresubjected to electrophoresis in a 0.7% agarose gel. As a results, it wasconfirmed that in connection with the plasmids of both strains,fragments exhibiting the same mobility as the DNA fragments of about 0.5kbp obtained by the AatI-BamHI digestion of pUC19-SOD14 occurred only inthe digest obtained by both restriction endonucleases AatI and BamHI.One of these plasmid was named "pTJ102-SOD14".

The flow chart of the above procedures is shown in FIG. 7.

(ii) Preparation of transformed yeast

As a yeast strain, Saccharomyces cerevisiae AH22 [a leu2 his4 can1(cir⁺)] [furnished courteously from Dr. Kenichi Matsubara, theCytoengineering Center, Osaka University; disclosed in Japanese PatentLaid-Open No. 36699/1984; FERM P-8840] was employed. It was inoculatedon 3 ml YPD culture medium (2% polypeptone, 1% yeast extract, 2%glucose), cultured overnight at 30° C., and centrifuged to harvestcells. The cells were suspended in 1 ml of prepared protoplast solution[50 μg/ml zymolyase-100T (product of Seikagaku Kogyo Co., Ltd.), 100 mMcitrate buffer (pH 5.8), 10 mM EDTA, 1M sorbitol] and the resultantsuspension was left over at 30° C. for 1 hour. After collecting thecells, they were washed twice with 1 ml of 1M sorbitol solution whichcontained 10 mM CaCl₂. The thus-washed cells were then suspended in 0.1ml of the solution. About 5 μg (5 μl) pTJ102-SOD14 was added to a 50 μlaliquot of the suspension. The resultant mixture was allowed to stand atroom temperature for 15 minutes, followed by an addition of 1 ml PEGsolution [20%(w/v) PEG 4000, 10 mM Tris-HCl (pH 7.0), 10 mM CaCl₂ ]. Theresultant suspension was left over at room temperature for 15 minutes.After collecting the cells, they were suspended in 0.1 ml 1M sorbitoland then added to 5 ml of a melted regeneration agar [1M sorbitol, 3%agar, and "Leu minus COM" {0.17% yeast nitrogen base, 0.5% ammoniumsulfate, 2% glucose, 20 μg/ml adenine sulfate, 20 μg/ml L-arginine.HCl,20 μg/ml L-histidine.HCl, 20 μg/ml L-methionine, 20 μg/ml L-tryptophane,20 g/ml uracil, 30 μg/ml isoleucine, 30 μg/ml L-lysineHCl, 30 μg/mlL-tyrosine, 50 μg/ml L-phenylalanine, 150 g/ml L-valine}] which had beenmaintained at 45° C. The thus-prepared suspension was spread on a platefor its culture. From the 3rd day, a transformant equipped with theL-leucine non-auxotrophism became visible. This is attributed to theretention of plasmid pTJ102-SOD14 by the yeast strain AH22. Thistransformed yeast was named "AH22/pTJ102-SOD14". Similarly, in order toindicate a transformant obtained as a result of transformation of AH22by a certain plasmid, the transformant will hereinafter be designated as"AH22/name of plasmid employed". For example, AH22 harboring pAM82 isdesignated as "AH22/pAM82".

(iii) The transformed yeast AH22/pTJ102-SOD14 obtained in the procedure(ii) and gained the L-Leucine non-auxotrophism was cultured at 30° C. in100 ml Burkholder minimal medium [see, Toh-e A. et al. (1973) J.Bacteriol., 113, 727-738] which contained 20 μg/ml L-histidine and 20μg/ml L-tryptophane. In the logarithmic phase of growth, the cells wereharvested and after removal of the medium, 100 ml of a fresh medium wasadded. The fresh medium contained 20 μg/ml L-histidine and 20 μg/mlL-tryptophane. It also contained KH₂ PO₄ in an amount one fiftieth thatin the Burkholder minimal medium and was instead added with KCl in anamount equivalent to the reduced weight of KH₂ PO₄. TheAH22/pTJ102-SOD14, the culture medium of which had been changed, wascultured at 30° C. for further 24 hours and the resulting cells wereharvested. Following the procedure proposed by Goscin et al. Biochemicaet Biophysica Acta, 289, 276-283 (1972)], the recombinant h-SOD wasextracted and by ion exchange chromatography, the recombinant h-SOD wasobtained in its purified form. This procedure will hereinafter bedescribed.

After AH22/pTJ102-SOD14 cells which had been induced for expression byculturing the recombinant h-SOD under the condition of low inorganicphosphate concentration in the above-described manner were harvested andfrozen overnight at -20° C., they were thawed at room temperature. A0.1M NaHCO₃ solution was added in an amount of 1.9 ml per gram of wetweight and the resultant mixture was stirred thoroughly to suspend thecells. Furthermore, ethanol-chloroform (5:3 v/v) was added in an amountof 1.1 ml per gram of wet weight and the resultant mixture was shakenovernight at 30° C. By this processing, the recombinant h-SOD wasextracted out from the yeast cells. By centrifugation ("RPR 20 Rotor"manufactured by Hitachi, Ltd.; 10000 rpm, 25° C., 10 minutes), thepellet and supernatant were separated from each other and thesupernatant was transferred in a separate container. To per ml of thesupernatant, 300 mg of K₂ PO₄ was gradually added in its solid form andwas then dissolved. As the container, an Eppendorf tube having acapacity of about 1.5 ml was used. After allowing the solution to standat room temperature for about 40 minutes, it was subjected tocentrifugal separation at 15000 rpm and 4° C. for 1 minutes by a smallcentrifuge. Of the resulting two layers and the intermediate thin-filmlike substance, the upper layer was transferred in a separate Eppendorftube. The container enclosing the upper layer was frozen at -110° C. for20 minutes, followed by its centrifugation at 15000 rpm and -7° C. for10 minutes by the small centrifuge. Of the resulting two layers and theintermediate thin-film like substance, the upper layer was transferredin a separate Eppendorf tube. The tube was ice chilled, to whichice-chilled acetone was added in an amount equivalent to 0.75 times thevolume of the contents. The resultant mixture was stirred and its icechilling was continued approximately for further 5 minutes. Thecontainer was centrifuged at 15000 rpm and 4° C. for 10 minutes by asmall centrifuge and the resulting supernatant was thrown away. Thethus-obtained precipitate was dissolved in its entirety in 1 mldistilled water, followed by addition of 1 μl Zn--Cu solution (1M CuSO₄,1 M ZnSO₄) and 20 μl potassium phosphate buffer (pH 6.8). In order toremove a precipitate which occurred upon the addition of the Zn--Cusolution, the mixture was centrifuged at 15000 rpm and 4° C. for 1minute by the small centrifuge. The resulting supernatant wastransferred in a separate container. This supernatant was transferred ina "CF25" container (product of Amicon Company; centrifugalultrafiltration membrane) and centrifuged at 1500 rpm and 4° C. for 10minutes by using "TS-7 Rotor" (manufactured by Tomy Seiko K.K.).Further, 1 ml distilled water was added to the "CF25" container andcentrifugation was conducted again under the same conditions. 300 μldistilled water was again added to the "CF25" container to wash offcomponents which were not allowed to pass through the filtrationmembrane. The washing was transferred to a separate container. Thewashing was transferred in a dialysis tube and was then dialyzedovernight at 4° C. against 500 ml 2.5 mM Na-K-phosphate buffer, whichhad been prepared by mixing 0.5M KH₂ PO₄ solution and 0.5M Na₂ HPO₄solution at pH 6.5 (room temperature) and then diluting the resultantmixture with distilled water. The inner solution of the dialysis tubewas taken out and after filtering it through "Column Guard SJHV 004NS"(product of Millipore Corporation), the recombinant h-SOD wasfractionated by the method which employed an anionic exchange material.Namely, the recombinant h-SOD contained in the filtrate was adsorbed ona "Mono Q 5/5 column" (HPLC column manufactured by Farmacia AB) whichhad been equilibrated by the above-mentioned 2.5 mM Na-K-phosphatebuffer. After washing off unadsorbed components with the 2.5 mMNa-K-phosphate buffer, the concentration of the Na-K-phosphate bufferwas changed linearly from 2.5 mM to 15 mM to elute the recombinant h-SODfrom the column. The thus-collected recombinant h-SOD was detected as anactive band at the same electrophoretic position as the native h-SODobtained from human red blood cells (product of Sigma Company) which hadbeen subjected as a control to electrophoresis in accordance with theactivity staining method employing an agarose gel film and described inthe procedure (v) of Example 1. Results are shown in FIG. 8 (righthandside: the recombinant h-SOD containing Ser at the 111-site of the aminoacid sequence and acetylated at the N-terminus thereof; lefthand side:h-SOD obtained from human red blood cells).

In the h-SOD cloning plasmid pTJ102-SOD14, the h-SOD gene is inserted inthe downstream of the pho5 promoter derived from pAM82. Thetranscription of the m-RNA, which contained the h-SOD gene, is effectedunder the control of pho5 promoter. It has been known that theactivities of the pho5 promoter are exhibited, for example, depending onthe concentration of inorganic phosphates in a culture medium in thecase of the combination of the pho5 promoter and AH22 strain[Miyanohara, A., et al., Proc. Natl. Acad. Sci. U.S.A., 80, 1-5 (1983)].Accordingly, similar inducible expression is also contemplated in thisexample. As a matter of fact, the expression control which depended uponthe concentration of inorganic phosphates was also observed in the caseof AH22/pTJ102-SOD14. The same strains were cultured in the same mannerin culture media having different inorganic phosphate concentrations.The cultured cells were subjected to zymolyase treatment and ultrasoniclysing. Thereafter, the amount of h-SOD derivative in the total cellextract from the same volume of each culture was measured by EIA [EnzymeImmunoassay; A specific antibody against Zn- and Cu-type SOD, which hadbeen obtained from human blood, was used.]. Results are summarized inthe following table.

    ______________________________________                                                  Strain                                                              Medium      AH22/pAM82   AH22/pTJ102-SOD14                                    ______________________________________                                        +His, +Trp, +Pi                                                                           --           4.2                                                  medium                                                                        +His, +Trp, 3.5          295                                                  low Pi                                                                        medium                                                                        ______________________________________                                    

The above values are data obtained by EIA with respect to theircorresponding cultured cell brothes of the same volume. The value 3.5 inthe table is considered to be the background value. It is alsoappreciated from the above example that the reduction to theconcentration of inorganic phosphate induced the expression of SOD.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the inventionas set forth herein.

We claim:
 1. A DNA sequence encoding a mutein of human Cu/Zn superoxidedismutase of 153 amino acids, the amino acid at least one of the 6 and111 position encoded by said DNA being serine or alanine.
 2. The DNA ofclaim 1, wherein said DNA is that of FIG. 1.